The present invention relates to a positioning device, an electronic instrument, and a filter passband changing method.
The global positioning system (GPS) is widely known as a positioning system which utilizes a satellite. The GPS is utilized for a car navigation system and the like. When locating a position using the GPS, a received signal (including a signal converted into an intermediate-frequency signal using a superheterodyne method) obtained by receiving a GPS satellite signal transmitted from a GPS satellite is filtered to remove unnecessary frequency components, and the GPS satellite signal is then acquired and tracked.
Specifically, the phases of a C/A code and a carrier frequency contained in the GPS satellite signal are acquired by performing a correlation process on the received signal which has passed through the filter and a pseudo-generated C/A code replica (code replica) to acquire the GPS satellite signal.
The GPS satellite signal is tracked by tracking the phase of the C/A code utilizing a code loop such as a delay locked loop (DLL), and tracking the phase of the carrier frequency utilizing a carrier loop such as a phase locked loop (PLL).
As related technology, JP-A-5-157826 discloses technology which changes the passband of the filter before and after establishment of synchronization with the GPS satellite signal.
According to the technology disclosed in JP-A-5-157826, frequency synchronization is facilitated by increasing the passband of the filter before frequency synchronization is established. On the other hand, if the passband of the filter is widened to a large extent, the number of noise components other than the GPS satellite signal increases, whereby the signal reception sensitivity decreases due to deterioration in S/N ratio.
According to the technology disclosed in JP-A-5-157826, the passband of the filter is decreased after frequency synchronization has been established. On the other hand, if the passband of the filter is narrowed to a large extent, the spread spectrum modulated GPS satellite signal is partially lost, whereby the positioning accuracy deteriorates. Specifically, since the positioning accuracy and the reception sensitivity have a trade-off relationship, changing the passband of the filter taking into account the presence or absence of establishment of frequency synchronization does not necessarily implement control which is appropriate for the reception environment while ensuring positioning accuracy and reception sensitivity.